Rudder Trim

[Bryan Cotton]

Thanks Graeme! I am going to set mine up to fly from either seat. I'll share what I come up with. I hadn't thought of under the seat. My boots are also fastened with velcro.

[GraemeSmith]

I also use a bungee but I use it as a supplement to the elevator trim.

In my first picture you can see a white bungee going up and down under the seat pan. Builder told me that was an "anti-flutter/damper" for the elevator - but it would also help with the trim. My trim is manual.

[Skippydiesel]

Hi Graeme,

You are lucky that you have so many passengers/copilots, that you must configure your Sonex for the load.

In my case I expect (based on history) to probably fly alone for 90+% of the time. I trimmed my last aircraft to cruise hands/feet off with just me on board. I had a pilot adjustable lateral trim bungee, that only acted on the right aileron rod. Very KISS and very effective in trimming for passenger weight.

I will be configuring my Sonex in the same way.

I cant see how trimming for an equal left/right load will be of any benefit UNLESS you intend having lateral trim on both ailerons - more complexity.

[bvolcko38]

Just a dumb question.... Could the rudder trim issue be better addressed by some engine thrust line adjustment?

[Bryan Cotton]

Just a dumb question.... Could the rudder trim issue be better addressed by some engine thrust line adjustment?

Possibly - but that would 1) be a lot of work and 2) mess up the prop/cowl alignment (which would also generate a lot of work.)

My trim is much like Graeme's:

So with an Aerovee - it is still left rudder on climb and right rudder on dive - but in level flight - I can take my feet off.

I am going to wait until I have the flaps/roll trim squared away before I tackle rudder trim. Mine is pretty close in cruise. I don't have springs on my pedals so I could add a single spring or bungee.

[GraemeSmith]

I cant see how trimming for an equal left/right load will be of any benefit UNLESS you intend having lateral trim on both ailerons - more complexity.

Well if I was to fly right seat on my own - I'd just hook the bungee up on the other side.

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What follow is OPINION based on some scenarios I have seen with planes that were not rigged correctly......

As a general principle - I don't like the idea of putting a plane "out of rig" to get it to fly straight. "Evenly loaded" Fore and aft and athwartships - control surfaces should be symmetrical or even with each other and the plane should fly straight - hands off, (Well as near as it will!). Trimming the control surfaces should then be used to account for loading conditions.

Rigging a plane to fly straight when unevenly loaded by tweaking control surfaces to not be symmetrical or even with each other - means when controls are "relaxed" or "centered" you start to introduce unequal control inputs, potential adverse yaws and flying in a slight skid or slip.

Now it might seem that this is no different than centering and then trimming. But it's not.

If you have rigged a plane to fly straight by having it skid or slip slightly - you have introduced an unknown element when flying on the edge of the flight envelope. The plane might stall, spin or behave unpredictably. You might not intend to fly on the edge of the envelope - but during a wake encounter and upset recovery - you want the plane as aerodynamically "in rig" as possible. No surprise effects.

While YOU might not have a problem with this slightly out of rig condition - the next owner of your aircraft might be in for a nasty surprise.

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End of opinion.

Let's look at the Cessna 100 series aircraft for a moment. Because with both older C150 and 172's I've seen this Frankenstein rigging situation occurs quite frequently - to the point the planes get dangerous. This one is a TRUE story:

The first clue was a trim tab applied to an aileron to correct a "wing heavy" condition. This is a major alarm bell - it should not be needed. The new owner - a low time freshly minted PPL who was time building reported the plane was dangerous in a power off stall. It rolled into an incipient spin virtually every time. It was not his low time. He got bac in the school plane and could stall straight and level no problem. Power on stalls almost guaranteed an over the top spin entry. When visually flying straight and level with light and apparently even control forces - the ball showed the plane was skidding. The plane's logs showed a number of attempts to fix the issue over the years and when we got to the root of it - it was mistake building on mistake building on mistake - each attempting to "fix" the plane. The advice was to stop flying the plane till it could be rerigged by someone who actually knows how to rig a C100 series.

So:

The empennage was not twisted from riding out a big storm in the tie downs.

The Turn Coordinator was correctly fitted - ball centered - when the plane was level on the ground.

Principle adjustment points are main wing root incidence on adjustable cams to vary each wing's AoA, bellcranks, tied rods and rigging tension for flaps, ailerons, elevators and rudder. Rudder tab (if fitted - it's an option). You also have to pay attention to "rudder springs" under the floor which center the rudder rigging in flight and make sure as you touch down the nosewheel is tracking straight - this is also achieved with a "cam" action on the extended nose strut that locks the wheel straight and unlocks as weight is applied to the compressing strut. Because the rudder pedals steer the nosewheel and work the rudder - those springs can skew the rudder. If one gets out of tension with the other - sitting on the ground - all can seem well - but in the air you get a different story.

Long and the short the experienced rigger removed the aileron tab and "let off" all the rigging. Then following Cessna's carefully choreographed sequence - "built up" the rigging situation from first principles till the plane measured "straight" and "in rig". Just about every single parameter of the rigging "as found" was completely different when rigged "as should be". Plane flew much better, stalled more or less gracefully - though was still dropping a wing more than was normal, and was still skidding a touch.

Back to the rudder springs - fit new just to be sure everything was equal - still didn't solve the problem.

A double check on the steering tie rods to check they were equally adjusted and........... They were not. But in a perverse way. The rods are actually supposed to be different lengths. They are different part numbers. These steering rods were the same length. One was the wrong part. When adjusted on the ground to steer straight - the wrong rod was applying some rudder in the air. Fit correct part and the whole plane was back in rig and flying correctly.

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Near as can be determined - WAY back in the logs - the plane was not steering straight on the ground. The mechanic at the time fitted another steering rod (assumed NOW to be the wrong one) and got the plane steering straight but she was now out of rig in the air - and every subsequent attempt to re-rig the plane only involved one parameter and just built worse on worse till someone fitted an aileron trim tab and the plane was downright dangerous ongoing.

Ground up re-rig from first principles putting everything where it should be - and the plane was docile and sweet to fly.

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YMMV

[Skippydiesel]

All very well articulated Graeme BUT unlike a factory build where jigs and sophisticated fabrication systems are the norm, ensuring a uniform end result, building from a kit or plans can & does result in unintended variations, that may not prevent the aircraft from flying but even when carefully rigged, according to Sonex instructions, results in different handling characteristics.
I am still working through my aircrafts anomalies - non major but negatively impact on my enjoyment of the aircraft.

[GraemeSmith]

a factory build where jigs and sophisticated fabrication systems are the norm, ensuring a uniform end result,

Your comment made me smile. I know some old timers from back in the day at Cessna. They will tell you - there were SOME jigs - but when they were cranking 150/2's and 172's - the planes were essentially hand built. You can't take a skin from (say a 1965 built) 150 and fit it to another. If nothing else - none of the rivet holes will REMOTELY line up. And the skins will often not fit for curvature. Hence when repairing - you have to start with a new skin and make it fit. Or have a whole component assembly from the junk yard with a chance the attachment points will line up.

They only did an actual Weight and Balance for every tenth plane off the line - as long as it was close enough to the one from ten previously - the ones in between got copy W&B - signed off as if the plane had actually been weighed.

Like car doors back in the day - the tolerances were good for their time and the budget but involved lots of shims and big gaps. Unlike modern cars where the gaps are tiny and doors and jigged and welded on by robots.

I'll hazard that a patient and thorough home builder will produce a more accurately/symmetrically built plane than those Cessna's back in the day. It may not be the same as the next Sonex over by the next builder over - but I'll hazard most modern home built will be built "truer".

Some speculation to the above......... But the bits about factory practices are true as told to me by those who were there.

[Skippydiesel]

I believe you but that was "back in the day" - what now?

Todays customer has much higher (quality) expectations from their car and I would guess (not being in the market) for a new aircraft/car. In large part this has been drive by Gov.legislation/oversight, more precise/repeatable production/fabrication techniques, etc.

[Skippydiesel]

I believe you but that was "back in the day" - what now?

Todays customer has much higher (quality) expectations from their car and I would guess (not being in the market) for a new aircraft/car. In large part this has been drive by Gov.legislation/oversight, more precise/repeatable production/fabrication techniques, innovations in materials, etc.